I need to encrypt a large file, primarily to stop other people bypassing my application and running the file using basic windows apps. As the file can reach 650MB's this takes a long time to encrypt.

Is there anyway that I can encrypt just a part of this file, so as to make it unreadable ?

I have looked at many encryption apps but they all take ages. I don't know if this partial encryption is possible, so any thoughts on this subject appreciated.

Regards

Mike